The invention relates to a process for detecting and indicating an anomalous thermal stress condition of a heating surface made from glass ceramic or a comparable material, in particular a glass ceramic surface, as well as a device for performing the process in a cooking area with a glass ceramic cooking surface.
Heating surfaces made from glass ceramic or a comparable material are used, for example, as wall or ceiling radiators, heat exchangers, or other large-surface heating devices, which can be heated in any way.
Electrically or gas-heated cooking areas or individual burners, whose heating surface consists of glass ceramic, are now of special interest. Cooking areas of this type are generally known and have already been described many times in the patent literature. Heating of the heating zones of these cooking areas takes place in this case by heating elements, placed below the glass ceramic cooking surface, e.g., electrically operated contact heating elements, radiant heating elements or gas burners. Further, induction cooking areas are also known.
In the known household cooking areas, the heat output for the heating elements is permanently adjusted by the presetting of the user or electronically, electromechanically or, with gas stoves by valves, purely mechanically controlled by a selectable time program. Corresponding controls are described, for example, in patent specification DE-PS 3 639 186 A1.
It is known to heat, with several heating elements, cooking zones (without narrowing the meaning, the term cooking zone instead of heating zone is used below in connection with glass ceramic cooking surfaces) of a glass ceramic cooking area, which exhibit a sizable diameter, for example, to heat pots with sizable diameters and/or nonround, for example, oval bottoms. It is also known, besides the permanent heating elements constantly in operation, to use so-called auxiliary heating elements, which are actuated only in the boiling phase, to achieve an accelerated heating of the cooking zone. The geometric arrangement of the heating elements below a heating zone then is usually matched to the geometry of the cookware.
Thus, for example, a hot plate with two heating circuits, concentric to one another, is described in DE-OS 33 14 501 A1, in which the outer heating circuit is designed as an auxiliary heating element.
DE-PS 34 06 604 relates to a heating device, in which the cooking zone is heated by high-temperature and normal-temperature radiant heating elements. The heating elements in this case are placed so that the cooking zone is divided into two zones, concentric to one another, and the inner zone can be heated only by the high-temperature radiant heating elements usable preferably as auxiliary heating elements in the boiling phase and the outer zone by the normal-temperature radiant heating elements. A comparable arrangement of several radiant heating elements in the area of a cooking zone is also to be found in U.S. Pat. No. 4,639,579.
A heating device with a gas burner, which exhibits two burner chambers, independent of one another and able to be actuated with gas, which, e.g., can delimit zones, concentric to one another, in the cooking zone area, is described in U.S. Pat. No. 4,083,355.
In the glass ceramics usually used, the maximum operating temperatures are to be limited to 700.degree. C. To avoid overheating the glass ceramic heating surface, therefore as a rule so-called protective temperature limitation devices, e.g., a bar expansion switch placed between the heating elements and the glass ceramic surface, are used, which usually turn off the heating element completely when a specific maximum temperature is exceeded or reduce its output. After passing through a hysteresis, the full heat output is again turned on. A bar expansion switch, for example, which suitably operates at two different temperatures, is known from DE-OS 3 314 501.
From German patent specification DE-PS 21 39 828, it is known that glass, glass ceramic or similar materials have an electrical conductivity dependent on the temperature, so that temperature-measuring resistances with steep resistance-temperature characteristics, similar to the known NTC resistances, can be produced from them by applying strip conductors, e.g., made from noble metals.
This type of temperature sensors is used for this purpose in DE-OS 37 44 372 in connection with the corresponding wiring, to replace the above-mentioned protective temperature limitation device completely. For this purpose, in each cooking zone two strip conductors each, parallel to one another, which each delimit a strip-like glass ceramic resistance, are applied along a half diameter on the glass ceramic cooking surface.
Experience has shown that anomalous thermal stresses in glass ceramic cooking surfaces result mostly from using inferior cookware or operating errors.
Thus, e.g., in cookware with uneven support surfaces, a locally varying removal of heat takes place in the cooking zone. By carelessness, empty cookware can cause still higher temperature/time stresses for the glass ceramic. Pots with too small diameters as well as inadvertently improperly placed, i.e., pots which are not centered, cause further extreme stresses. In these cases, the cooking zone in the areas not covered by the pot is overheated. The surface temperature of the glass ceramic in such cases can be considerably above the temperatures measured in the operation without a pot. Temperature increases of up to 200 K above the surface temperature in operation without a pot are possible.
These anomalous thermal stresses in the area of the cooking zones can add up to high temperature/time stresses over time and can bring about the destruction of the cooking surfaces. Extremely high temperatures can damage the superposed cookware and also the glass ceramic cooking surfaces. Pot enamel, for example, can melt in the case of steel enamel cookware which is inadvertently placed empty on a glass ceramic cooking surface. Also, aluminum cookware left on the cooking surface while empty can damage the glass ceramic surface by melting aluminum.
Since in practice, both inferior or unsuitable cookware is used and the above-mentioned operating errors occur, the maximum surface temperature in the operation without a pot has to be limited. For the same reason, the specific output density of the heating elements, relative to the surface of the heated zone, is now limited to about 7 watt/cm.sup.2.
The known devices for temperature monitoring and limitation have the drawback that as soon as overheating is detected in the area of the temperature sensors, the power supply for the heating elements in the entire cooking zone is interrupted or reduced. This leads to considerably longer boiling periods, although, as, e.g., in the case of an incorrect placement of the cookware, the reason for the anomalous thermal stress condition could be easily eliminated by the user.